1. Field of Invention
This invention relates to medical diagnostic and monitoring systems. More specifically, this invention is directed to a system and method for accurately monitoring the cardiovascular state and physiologic condition of a living subject.
2. Description of Related Art
Blood-pressure measurement devices are commonly used to diagnose certain ailments, such as arteriosclerosis, and to monitor a living subject's cardiovascular or physiologic state.
A blood-pressure measurement device typically measures the living subject's blood pressure indirectly with the use of a cuff that is wrapped around a portion of the living subject. The cuff applies pressure to the living subject. The living subject's blood pressure is then measured using a well-known oscillometric method, which is based on detecting changes in the amplitude of a heartbeat-synchronous pulse wave as the pressure applied by the cuff is gradually decreased.
The accuracy of this indirect blood-pressure measurement method is dependent upon a number of design factors, such as the size of the cuff relative to the circumference of the appendage that the cuff is wrapped around, the accuracy of the pressure-sensing devices used by the blood-pressure measurement device, and the performance of the algorithm used to calculate the living subject's systolic, diastolic and mean blood pressures.
An indirect blood-pressure measurement using an occlusion cuff typically takes twenty to sixty seconds. An important factor that influences the accuracy of all indirect blood-pressure measurement methods that utilize an occlusion cuff is the assumption that the living subject's blood pressure remains constant during the measurement. However, a living subject's physiologic state can change significantly over the indirect blood-pressure measurement time, particularly if the living subject is a surgical or critical care patient that is suffering from advanced cardiovascular disease, respiratory disease, kidney disease or blood loss. The change in the living subject's cardiovascular state may be due to changes in heart rate, cardiac output, vasomotor tone or circulating blood volume.
Changes in the living subject's blood pressure during the blood-pressure measurement will distort the amplitude envelope of the pressure pulses produced in the occlusion cuff as the pressure applied by the cuff passes through the blood-pressure pulse range. The distortion of the amplitude envelope will result in a shifting of the amplitude envelope features associated with the living subject's systolic, diastolic and mean blood pressures. This is particularly true for oscillometric blood-pressure measurement devices that analyze the entire amplitude envelope of the pressure pulses.
If the living subject's blood pressure is changing during the blood-pressure measurement, the time delay between the measurement of the subject's systolic blood pressure and the measurement of the subject's diastolic blood pressure results in an erroneously high measured pulse pressure or an erroneously low measured pulse pressure. As shown in FIG. 1, if the living subject's blood pressure is falling during the blood-pressure measurement, the measured systolic blood pressure S.sub.M is higher than the living subject's true systolic blood pressure S.sub.T when the living subject's measured diastolic blood pressure D.sub.M is measured. As a result, the measured pulse-pressure PPM is greater than the living subject's actual pulse-pressure PP.sub.T. If the living subject's blood pressure is rising during the blood-pressure measurement, the opposite effect occurs. In either case, the erroneous blood pressure values may mislead or confuse a living subject's caregiver.
Another phenomenon that can cause the living subject's blood pressure to change during the blood-pressure measurement is an abnormal heartbeat, commonly referred to as an arrhythmia. Arrhythmias are common in critically ill patients and result when the living subject's heart contracts earlier or later than normal. Arrhythmias can also result when the heart contracts with increased or decreased strength.
Arrhythmias can be produced by a number of abnormalities in the electromyocardial conduction system. The physiologic events that cause arrhythmias originate in the heart rather than the central nervous system.
Arrhythmias can be easily identified in an electrocardiogram by comparing the inter-beat interval time of successive heartbeats and the morphology of the electrocardiogram complex. However, arrhythmias are difficult to identify in other physiologic signals, such as an oscillometric pulse signal. This is because the oscillometric pulse signal is sensitive to environmental noise and motion artifacts, e.g., movement of the living subject during a blood-pressure measurement. Environmental noise or motion artifacts can produce pulses that are similar to the pulses produced by arrhythmias.
Most blood-pressure measurement devices exclude arrhythmias from the oscillometric blood-pressure measurement for two reasons. First, most blood-pressure measurement devices cannot reliably differentiate an arrhythmia from an environmental or motion artifact that should be deleted from the data set. Second, blood-pressure pulses produced by arrhythmias alter oscillometric pulse amplitudes. The altered oscillometric pulse amplitudes distort the amplitude envelope of the pulses used for measuring the living subject's blood-pressure.
In blood-pressure measurement devices that utilize a step-wise deflation cuff, multiple pulses are detected at each cuff pressure level until a non-arrhythmic pulse is detected. This method can lead to excessively long measurement times if the living subject's heart exhibits frequent arrhythmias. In blood-pressure measurement devices that utilize a continuous-deflation cuff, arrhythmic pulses are rejected and extrapolation is used to fill in the missing data point. This method results in a data set with lower resolution and a decrease in the accuracy of the blood-pressure measurement. The effect of arrhythmias on the accuracy of the blood-pressure measurement is dependent upon the relationship of the arrhythmia to the systolic, diastolic and mean blood-pressure detection points.
Other conditions can also result in less accurate or misleading blood-pressure measurements. For example, the ultimate accuracy of an indirect blood-pressure measurement is dependent upon the change in cuff pressure between detected heartbeats. If the living subject's heart rate is very low relative to the cuff's deflation rate, a large change in cuff pressure will occur between heartbeats. This reduces the accuracy of the blood-pressure measurement. In addition, if the living subject is in shock, the living subject's blood-pressure pulses, and the resulting measured oscillometric pulses, are too low for identification of the blood-pressure features in the amplitude envelope of the oscillometric pulses. If the living subject is restless, shivering or otherwise moving, there may be too many motion artifacts to make a blood-pressure measurement.
Most blood-pressure measurement devices make several attempts to measure the living subject's blood pressure before they sound an alarm. In this situation, the monitoring of the living subject's condition is interrupted while a user corrects the problem.